System for measuring impedance between a plurality of electrodes of a medical device

Abstract

The present disclosure is directed to measuring impedance across a plurality of electrode pairs. The disclosed systems and methods may simultaneously provide drive signals between electrode pairs and then sense the voltage signals that develop at the electrodes. Digital signal processing may be used to synchronously demodulate the voltage signal at each electrode to determine impedances at the electrodes. Each electrode pair may be driven at a unique frequency to allow for significantly increasing a number of electrode pairs and/or increasing drive current magnitudes. Synchronous demodulation allows the unique frequencies to be detected independent of each other while minimizing crosstalk. Typically, the drive frequencies are made orthogonal by setting the drive frequencies at harmonics of a common base frequency and measuring a response over an integer number of cycles. In an embodiment, quadrature demodulation may occur providing a real component for resistive impedance and an imaginary component for reactive impedance.

What is claimed is: 
     
       1. An electronic control unit for measuring impedance between electrodes of a connected medical device, comprising:
 a controller configured to:
 generate a plurality of drive signals each having a unique modulation frequency that is a harmonic of a common base frequency; and 
 simultaneously apply each of said plurality of drive signals across an individual pair of electrodes of the medical device; 
 
 a measurement circuit for measuring a composite response signal of said plurality of drive signals as applied to a plurality of individual pairs of electrodes of the medical device; 
 a demodulator configured to synchronously demodulate said composite response signal for each said unique modulation frequency, wherein for each said unique modulation frequency said demodulator multiplies said composite response signal with a demodulation signal having an identical frequency to said unique modulation frequency and a known phase that is offset from a phase of said unique modulation frequency; and 
 wherein said controller outputs an impedance value for each of said electrodes based on the synchronous demodulation of the composite response signal. 
 
     
     
       2. The control unit of  claim 1 , wherein said impedance value comprises a complex impedance value having a resistive impedance and a reactive impedance. 
     
     
       3. The control unit of  claim 1 , wherein said demodulator samples over a sampling period having an integer number of cycles for said common base frequency. 
     
     
       4. The control unit of  claim 1 , wherein said controller further comprises at least one digital to analog converter (DAC) for converting said plurality of drive signals from digital drive signals to analog drive signals prior to being applied across said plurality of individual pairs of electrodes. 
     
     
       5. The control unit of  claim 4 , further comprising at least a first numerically controlled oscillator (NCO) for generating said plurality of digital drive signals, wherein said digital drive signals are received by said at least one DAC. 
     
     
       6. The control unit of  claim 5 , wherein said demodulator further comprises at least a second numerically controlled oscillator (NCO) for generating demodulation signals. 
     
     
       7. The control unit of  claim 6 , wherein said controller, said first NCO and said second NCO and said demodulator are elements of a common field programmable gate array (FPGA). 
     
     
       8. The control unit of  claim 6 , wherein said controller is further configured to input each said unique modulation frequency to said first NCO and said second NCO. 
     
     
       9. The control unit of  claim 8 , wherein said controller is further configured to input a predetermined random phase offset to said first NCO and to said second NCO for each said unique modulation frequency. 
     
     
       10. The control unit of  claim 1 , wherein said controller is configured to:
 identify a number of individual pairs of electrodes for the medical device; and 
 adjust a current level of said drive signals based on said number of individual pairs of electrodes. 
 
     
     
       11. The control unit of  claim 10 , wherein said controller maintains a combined current level for said drive signals below a predetermined current threshold as a function of a root mean square of said number of individual pairs of electrodes times said current level. 
     
     
       12. The control unit of  claim 10 , wherein said controller is configured to selectively disable one or more of said individual pairs of electrodes. 
     
     
       13. The control unit of  claim 1 , wherein said controller is configured to: generate at least fifty of said drive signals each having said unique modulation frequency and each having a drive current below 20 micro-amps. 
     
     
       14. An electronic control unit for measuring impedance between electrodes of a connected medical device, comprising:
 a frequency source configured to generate a plurality of digital drive signals each having a unique modulation frequency that is a harmonic of a common base frequency; 
 at least a first digital to analog converter configured to receive said plurality of digital drive signals and output a plurality of analog drive signals, wherein said plurality of analog drive signals are simultaneously applied across a corresponding plurality of individual pairs of electrodes of the medical device; 
 at least a first analog to digital converter configured to receive analog responses from the electrodes and generate a composite digital response signal of said plurality of drive signals as simultaneously applied to the individual pairs of electrodes, 
 a demodulator circuit configured to:
 synchronously demodulate said composite response signal for each said unique modulation frequency; and 
 output an impedance value for said electrodes. 
 
 
     
     
       15. The control unit of  claim 14 , wherein said frequency source further comprises at least a first numerically controlled oscillator (NCO) for generating said plurality of drive signals, wherein each of the first NCOs is assigned a random phase offset. 
     
     
       16. The control unit of  claim 15 , wherein said demodulator circuit further comprises at least a second numerically controlled oscillator (NCO) for generating a plurality of demodulation signals, wherein each demodulation signal has:
 an identical frequency to said unique modulation frequency of a corresponding one of said plurality of drive signals; and 
 a known phase offset that compensates for the random phase offset assigned to each of the first NCOs plus phase delay between the first NCO and the first analog to digital converter. 
 
     
     
       17. The control unit of  claim 16 , wherein said first NCO and said second NCO are elements of a common field programmable gate array (FPGA). 
     
     
       18. The control unit of  claim 14 , wherein said frequency source is configured to generate at least fifty of said digital drive signals each having said unique modulation frequency and each having a drive current below 20 micro-amps. 
     
     
       19. A method for use in measuring impedance across a plurality of individual pairs of electrode of a medical device, comprising:
 generating a plurality of drive signals each having a unique modulation frequency that is a harmonic of a common base frequency; and 
 simultaneously applying each of said plurality of drive signals across an individual pair of electrodes of the medical device; 
 measuring a composite response signal of said plurality of drive signals as applied across a plurality of individual pairs of electrodes of the medical device; 
 synchronously demodulating said composite response signal for each said unique modulation frequency; and 
 outputting an impedance value for each of said electrodes. 
 
     
     
       20. The method of  claim 19 , wherein synchronously demodulation further includes sampling the composite response signal over a period having an integer number of cycles for said common base frequency. 
     
     
       21. The method of  claim 19 , further comprising:
 identifying a number of individual pairs of electrodes for the medical device; and 
 adjusting a current level of said plurality of drive signals based on said number of individual pairs of electrodes. 
 
     
     
       22. The method of  claim 19 , further comprising maintaining a sum of current levels for said drive signals below a predetermined total current threshold as a function of a root mean square of said number of individual pairs of electrodes times a current level of each said drive signal. 
     
     
       23. The method of  claim 19 , wherein generating said plurality of drive signals comprises generating at least fifty drive signals each having a drive current below 20 micro-amps.